Phospholipid metabolism was found to play a significant role in natural killer (NK) activity. Transmethylation and phospholipase A2 action were demonstrated to represent important changes in phosphatidylcholine metabolism observed during effector:target interaction and cell lysis. These changes in phospholipid metabolism, and others including effects on the CDP-choline pathway, induced by interferon and the tumor promotor phorbol myristate acetate (PMA), were studied in detail. Growth inhibitory activity (GIA) was studied in normal individuals and cancer patients. Interferon and PMA were found to exert effects in this system. A novel assay for GIA based on inhibition of alkaline phosphatase activity was developed. Efforts have been made to develop hybridomas producing monoclonal antibodies directed against cell surface components of human monocytes and natural killer cells.